Adaptive two-dimensional channel interpolation

ABSTRACT

A method and apparatus for improving channel estimation within an OFDM communication system. Channel estimation in OFDM is usually performed with the aid of pilot symbols. The pilot symbols are typically spaced in time and frequency. The set of frequencies and times at which pilot symbols are inserted is referred to as a pilot pattern. In some cases, the pilot pattern is a diagonal-shaped lattice, either regular or irregular. The method first interpolates in the direction of larger coherence (time or frequency). Using these measurements, the density of pilot symbols in the direction of faster change will be increased thereby improving channel estimation without increasing overhead. As such, the results of the first interpolating step can then be used to assist the interpolation in the dimension of smaller coherence (time or frequency).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/710,527 filed on Aug. 23, 2005 and U.S. ProvisionalPatent Application No. 60/722,744 filed on Sep. 30, 2005, which are eachhereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to Orthogonal Frequency Division Multiplexing(OFDM) communication systems, and more particularly to channelinterpolation with the use of pilot symbols.

BACKGROUND OF THE INVENTION

In wireless communication systems that employ OFDM, a transmittertransmits data to a receiver using many sub-carriers in parallel. Thefrequencies of the sub-carriers are orthogonal.

Channel estimation in OFDM is usually performed with the aid of knownpilot symbols which are sparsely inserted in a stream of data symbols.The attenuation of the pilot symbols is measured and the attenuations ofthe data symbols in between these pilot symbols are thenestimated/interpolated.

Pilot symbols are overhead, and should be as few in number as possiblein order to maximize the transmission rate of data symbols. It isdesirable that channel estimation in OFDM be as accurate as possiblewithout sacrificing bandwidth.

SUMMARY OF THE INVENTION

In one embodiment, there is provided a method comprising receivingchannel estimates for four pilot symbols in a scattered pilot pattern intime-frequency; calculating the channel response for the pilot symbolsin both a first direction and a second direction; determining whetherthe channel changes more slowly in one direction than the other; andinterpolating in the direction of slower channel change.

In some embodiments, the method of further comprises interpolating inthe direction of faster channel change.

In some embodiments, the step of interpolating in the direction offaster channel change is performed using the result from the step ofinterpolating in the direction of slower channel change.

In some embodiments, the channel changes are calculated by performing aninner products operation.

In some embodiments, the first direction is a time direction and thesecond direction is a frequency direction.

In some embodiments, the first direction is a frequency direction andthe second direction is a time direction.

In some embodiments, the scattered pilot pattern is a regular diamondlattice.

In some embodiments, the scattered pilot pattern is an irregular diamondlattice.

In some embodiments, the scattered pilot pattern is kite shaped.

In another embodiment, there is provided an OFDM receiver comprising:one or more receive antennas; the OFDM transmitter being adapted toreceive channel estimates for four pilot symbols in a scattered pilotpattern in time-frequency, calculate channel changes for the pilotsymbols in a first direction and a second direction, and interpolate inthe direction of slower channel change.

In yet another embodiment, there is provided a method of interpolationusing a set of four pilot symbols in a scattered pilot pattern intime-frequency wherein the set of four pilot symbols comprise first andsecond pilot symbols on a common sub-carrier frequency, spaced in time,and third and fourth pilot symbols transmitted on different sub-carrierson a common OFDM symbol period, the method comprising: determining afirst channel change between the first and second pilot symbols;determining a second channel change between the third and fourth pilotsymbols; determining which of the first and second channel change isslower; if the first channel change is slower, interpolating using thefirst and second pilot symbols to generate a channel estimate for thecommon sub-carrier frequency at the common OFDM symbol period, and thenusing the channel estimate in subsequent interpolations to determineother channel estimates; and if the second channel change is slower,interpolating using the third and fourth pilot symbols to generate achannel estimate for the common sub-carrier frequency at the common OFDMsymbol period, and then using the channel estimate in subsequentinterpolations to determine other channel estimates.

In yet another embodiment, a method of inserting pilot symbols into OFDMsub-frames for transmission by a plurality of transmitting antenna, theOFDM sub-frames having a time domain and a frequency domain, each OFDMsub-frame comprising a plurality of OFDM symbols, the method comprising:for each sub-frame, defining a set of at least two OFDM symbols none ofwhich are consecutive that are to contain pilot symbols; at eachantenna, inserting pilot symbols in each of the set of at least two OFDMsymbols in a scattered pattern that does not interfere with thescattered pattern inserted by any other antenna.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is a diagram of a single antenna perfect diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 2 is a flowchart of a method of performing adaptive interpolationin accordance with one embodiment of the present invention;

FIG. 3 presents simulation results for one example of adaptiveinterpolation;

FIG. 4A is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can used in accordance with an embodimentof the present invention;

FIG. 4B is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can used in accordance with an embodimentof the present invention;

FIG. 5A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 5B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 5C is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 5D is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 6 is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 7A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 7B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 7C is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 7D is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 8 is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 9A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 9B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 9C is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 9D is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 10A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 10B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 11A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 11A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 12A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 12B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention;

FIG. 13 is a block diagram of a cellular communication system;

FIG. 14 is a block diagram of an example base station that might be usedto implement some embodiments of the present invention;

FIG. 15 is a block diagram of an example wireless terminal that might beused to implement some embodiments of the present invention;

FIG. 16 is a block diagram of a logical breakdown of an example OFDMtransmitter architecture that might be used to implement someembodiments of the present invention;

FIG. 17 is a block diagram of a logical breakdown of an example OFDMreceiver architecture that might be used to implement some embodimentsof the present invention; and

FIG. 18 is a block diagram of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Channel estimation in OFDM is usually performed with the aid of pilotsymbols. More particularly, at an OFDM transmitter, known pilot symbolsare periodically transmitted along with data symbols. The pilot symbolsare typically spaced in time and frequency.

The variations in phase and amplitude resulting from propagation acrossan OFDM channel are referred to as the channel response. The channelresponse is usually frequency and time dependent. If an OFDM receivercan determine the channel response, the received signal can be correctedto compensate for the channel degradation. The determination of thechannel response is called channel estimation. The transmission of knownpilot symbols along with data symbols allows the receiver to carry outchannel estimation.

When a receiver receives an OFDM signal, the receiver compares thereceived value of the pilot symbols with the known transmitted value ofthe pilot symbols to estimate the channel response.

Since the channel response can vary with time and with frequency, thepilot symbols are scattered amongst the data symbols to provide a rangeof channel responses over time and frequency. The set of frequencies andtimes at which pilot symbols are inserted is referred to as a pilotpattern. In some cases, the pilot pattern is a diagonal-shaped lattice,either regular or irregular.

FIG. 1 is an example pilot pattern which can be used in accordance withone embodiment of the present invention. Pilot and data symbols arespread over an OFDM sub-frame in a time direction 120 and a frequencydirection 122. Most symbols within the OFDM sub-frame are data symbols124. Pilot symbols 126 are inserted in a diamond lattice pattern. In theillustrated example, the diamond lattice pattern in which each encodedpilot symbols are inserted within the OFDM sub-frame is a perfectdiamond lattice pattern as illustrated by pilot symbols h₁, h₂, h₃ andh₄.

A two dimensional interpolator is used to estimate the channel responseat point h which is between four points of known channel response, i.e.pilot symbols h₁, h₂, h₃ and h₄. Point h can then be used as anadditional point from which the receiver can carry out channelestimation. The use of point h would, of course, not add any overhead tothe OFDM signal.

The channel interpolation scheme is adaptive, i.e. it is a scheme whichcan adapt to varying conditions in the channel. The following formulapresents a particular example of adaptive two-dimensional (timedirection and frequency direction) interpolator to calculate point h:

h(i, j)=w ₁(i, j)h ₁ +w ₂(i, j)h ₂ +w ₃(i, j)h ₃ +w ₄(i, j)h ₄

where

w ₁(i, j)+w ₂(i, j)+w ₃(i, j)+w ₄(i, j)=1

In this case, the two dimensional channel interpolation can be viewed asthe sum of two one-dimensional interpolations.

The weights w_(k)(i,j) may be adapted to coherence time and frequency ofthe channel. In some embodiments, if the channel coherence is less inthe time direction than it is in the frequency direction, then h wouldbe calculated using the following formula:

h(i, j)=w ₁(i, j)h ₁ +w ₂(i, j)h ₂ +w ₃(i, j)h ₃ +w ₄(i, j)h ₄

where

w₃(i,j)=0

w₄(i,j)=0 and

w ₁(i, j)+w ₂(i, j)=1

Alternatively, if the channel coherence is greater in the time directionthan it is in the frequency direction, then h would be calculated usingthe following formula:

h(i, j)=w ₁(i, j)h ₁ +w ₂(i, j)h ₂ +w ₃(i, j)h ₃ +w ₄(i, j)h ₄

where

w₁(i,j)=0

w₂(i,j)=0 and

w ₃(i, j)+w ₄(i, j)=1

In another embodiment, the weights in both directions (time andfrequency) are adaptively changed according to the channel coherence inthe time and frequency directions as follows:

h(i, j)=c_(time) w ₁(i, j)h ₁ +c _(time) w ₂(i, j)h ₂ +c _(freq) w ₃(i,j)h ₃ +c _(freq) w ₄(i, j)h ₄

c _(time) +c _(freq)=1

w ₁(i,j)+w ₂(i,j)+w ₃(i,j)w ₄(i,j)=1

According to one embodiment, the sequence of interpolation is adapted tothe coherence of the channel. One way to achieve adaptive interpolationis to divide the interpolation into two one-dimensional steps as shownin the flowchart illustrated in FIG. 2:

i. at step 210, calculate the channel changes in both time and frequencydirections and determine in which direction the channel changes faster;

ii. at step 220, perform one-dimensional interpolation in the directionwith slower channel change to calculate h; and

iii. at step 230, using h, perform one-dimensional interpolation in thedirection with faster channel change.

The method of adaptive interpolation set out above takes advantage ofthe fact that interpolated results from the direction of largercoherence time/frequency is more reliable, and hence is interpolatedfirst. The calculation of h will effectively increase the density ofpilot symbols in the direction of faster change thereby improvingchannel estimation without increasing overhead. As such, the results ofthe first interpolating step can then be used to assist theinterpolation in the dimension of smaller coherence time/frequency.

In general, there are at least three ways to evaluate the channel changebetween two pilots, including:

i. Euclidean distance. One problem with Euclidean distance, however, isthat it is not sensitive to phase change;

ii. Phase change. One problem with phase change, however, is computationcomplexity; and

iii. Amplitude change. One problem with amplitude change, however, isthat it is insensitive to phase change.

In light of these drawbacks a way to measure channel change so as totake both amplitude change and phase change into account, while at thesame time keeping the computation complexity to a minimum, is desirable.According to an embodiment of the invention, therefore, a way of usingthe inner products of the two pilot assisted channel estimates as ameasurement of channel change is shown below.

_(time) =

h ₃ ,h ₄

=|h ₃ ∥h ₄|cos(θ_(3,4))

_(freq) =

h ₁ ,h ₂

=|h ₁ ∥h ₂|cos(θ_(1,2))

_(time) denotes channel change in the time direction.

_(freq) denotes channel change in the frequency direction.

The term “<h_(n)·h_(m)>” denotes the inner product of h_(n) and h_(m).

The term “|h_(n)|” denotes the magnitude of the vector h_(n). Ifh_(n)=a+bi then |h_(n)|=sqr(a²+b²).

The term “cos(θ1,2)” denotes the cosine of the difference in anglebetween h_(n) and h_(m):cos(θn,m)=cos(θn−θm). If h_(n)=a+bi then θn canbe calculated as θn=tan⁻¹(b/a).

The vector h_(n) can be represented as h₁=|h₁|e^(iθn), or as h_(n)=a+bi,where

a=|hn|cos(θn), and b=|hn|sin(θn).

When the amplitude changes linearly between the two channel estimates,the maximum

is achieved when |h₁|=|h₂| in frequency and |h₃|=|h₄| in time.

Hence the more the channel changes, the smaller the Λ, regardlesswhether this change is in amplitude or phase. The inner product is ableto solve phase ambiguity. When |θ|>π/2 (which rarely occurs), cos(θ)becomes negative, and hence smaller. An inner product may then becomputed, which requires two real multiplications and one real addition,and the result is therefore a real number.

Referring again to FIG. 1, what follows is an example of the adaptiveinterpolation method.

Assume:

h ₁=0.4423−1.0968i

h ₂=−0.0051−0.1484i

h ₃=0.1258−0.3413i

h ₄=0.3958−0.5883i

The central point, known from a simulation, has the value ofh=0.2859−0.4224i.

The inner product is then calculated as follows:

h ₁ ·h ₂

=0.1605

h ₃ ·h ₄

=0.2506

where

h₁·h₂

=denotes the inner product of h₁ and h₂.

If h₁=a₁+ib₁ and h₂=a₂+ib₂ then the inner product can be calculated as

h ₁ ·h ₂

=a ₁ a ₂ +b ₁ b ₂.

Alternatively, <h₁·h₂>=|h₁| |h_(2| cos(θ) ₂−θ₁).Since

h₁·h₂

<

h₃·h₄

, the channel changes faster in the h₁/h₂ direction.h is then estimated in both the frequency and time directions:

{tilde over (h)} _(h1,h2)=0.5(h ₁ +h ₂)=0.2186−0.6226i

{tilde over (h)} _(h3,h4)=0.5(h ₃ +h ₄)=0.2608-0.4648i

Compared with the known h, obviously {tilde over (h)}_(h3·h4) provides abetter estimate than {tilde over (h)}_(h1,h2); hence {tilde over(h)}_(h3,h4) can be used to improve the channel interpolation quality inthe h₁/h₂ direction.

Note that there is no requirement that h be the middle point equidistantfrom h₁, h₂, h₃ and h₄.

In the example above, the interpolation sequence was determined to be:

i. interpolate between the two pilots in the time direction first tocalculate h, and

ii. use h and/or one or both of the two pilots to interpolate in thefrequency direction.

Of course, if the initial calculation used to determine which channeldirection changes faster determines that the h₃/h₄ direction changesfaster, then the interpolation sequence will be:

i. interpolate between the two pilots in the frequency direction firstto calculate h, and

ii. use h and/or one or both of the two pilots to interpolate in thetime direction.

Once h is calculated, any one of a number of conventional channelestimation techniques can be used. Such channel estimation techniquestypically consist of two steps. First, the attenuations at the pilotpositions are measured. This measurement is calculated using theformula:

${H\left( {n,k} \right)} \equiv \frac{Y\left( {n,k} \right)}{X\left( {n,k} \right)}$

where X(n,k) is the known pilot symbol, and Y(n,k) is the received pilotsymbol.

These measurements are then used to estimate (interpolate) theattenuations of the data symbols in the second step. Persons skilled inthe art will appreciate that such channel estimation techniques include,but are not limited to, linear interpolation, second orderinterpolation, maximum likelihood (least square in time domain), linearminimum square error and others.

In another embodiment, a “majority vote” is used to determine theinterpolation sequence for all the “diamonds” across the frequencydomain. This means that there are several calculations performed alongthe frequency direction for the channel change. Some results willindicate there is more change in time, while other results indicatethere is more change in frequency. The “majority vote” option means thechoice whether to interpolate first in the time direction or thefrequency direction is arrived at by assessing the majority of theresults. For example, if the majority of the results indicate that thechannel changes faster in the time direction, then interpolation isfirst performed in the frequency direction, and then in the timedirection. If the majority of the results indicate that the channelchanges faster in the frequency direction, then interpolation is firstperformed in the time direction, and is then performed in the frequencydirection.

In accordance with an embodiment of the invention, FIG. 3 presentssimulation results for the adaptive interpolation method describedabove. The results show the benefit of adaptive interpolation whenchannel changes slower in the time direction when UE speed is low, andslower in the frequency direction when UE speed is high. The curve of“ideal channel” is of the case with clean known channel, i.e. with nointerpolation loss and additive noise. As shown this approach recoupsmost of the interpolation loss. The results were obtained with themajority vote option described above.

It is not necessary that there be a regular diamond shaped pilot patternin order to use the adaptive interpolation method described above. Forexample, an irregular diamond shaped pilot pattern can be used inaccordance with other embodiments of the present invention, such as thescattered pilot patterns shown in FIGS. 4A to 11. In FIGS. 4A to 11, thenumber of OFDM symbols per Transmission Time Interval (TTI) is oddinstead of even. In some embodiments, the scattered pilot patterns canbe generated by more than one antenna such as is shown in FIGS. 5, 7, 9,10, 11 and 12.

In general, the adaptive interpolation method works with all “staggered”pilot patterns which describes all shapes other than a square, whichdoes not work. A perfect diamond shape, which is the most favourableshape, is a special case of a staggered pilot pattern. Another exampleof a pattern which would work is a “kite” pattern where the pilots arespread further apart in one direction than the other.

More generally, in FIGS. 4A to 11, in each sub-frame, pilots aretransmitted by part of the sub-carriers in at least two non-contiguousOFDM symbols by at least one transmit antenna. The pilot sub-carriers inthe first OFDM symbol and the second OFDM symbol are staggered in thefrequency domain. In FIGS. 5A, 5D, 7A, 7D, 9A and 9D, pilot symbols fromall transmit antennas are transmitted through the same non-contiguousOFDM symbols. This arrangement will save the terminal power since onlytwo OFDM symbols are coded to obtain the channel information.

FIG. 4A is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can used in accordance with an embodimentof the present invention. The overhead associated with this pilotpattern is 1/28 per antenna. Pilot and data symbols are spread over anOFDM sub-frame in a time direction 420 and a frequency direction 422.Most symbols within the OFDM sub-frame are data symbols 424. Pilotsymbols 426 are inserted in an irregular diamond lattice pattern. Inthis embodiment, an OFDM sub-frame comprises eight sub-carriers 428 andseven OFDM symbols 430.

As with the scattered pilot pattern in FIG. 1, there is first performeda calculation of the channel changes in both the time direction and thefrequency direction and a comparison is made as to which direction thechannel changes faster. One-dimensional interpolation is then performedin the direction with slower channel change. One-dimensionalinterpolation is then performed in the direction with faster channelchange.

FIG. 4B is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can used in accordance with an embodimentof the present invention. Though similar to FIG. 4A, in this case one ofthe pilots in each diamond lattice is offset by one OFDM symbolposition. Thus, the adaptive interpolation method does not require thatthe scattered pilots line up in either or both of the time direction andthe frequency direction. In the case of staggered pilot patterns wherethe pilots do not line up in either the time direction, the frequencydirection, or both, it is more accurate to refer to the “h₁/h₂direction” and the “h₃/h₄ direction” rather than the time direction andthe frequency direction.

FIG. 5A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention. The overhead associated with thisscattered pilot pattern is 1/28 per antenna. In this embodiment, an OFDMframe comprises eight sub-carriers 528 and seven OFDM symbols 530.

Pilot and data symbols are spread over an OFDM frame in a time direction420 and a frequency direction 522. Most symbols within the OFDM frameare data symbols 524. Pilot symbols 526 are inserted in an irregulardiamond lattice pattern.

As with the scattered pilot pattern in FIG. 1, there is first performeda calculation of the channel changes in both the time direction and thefrequency direction and a comparison is made as to which direction thechannel changes faster. One-dimensional interpolation is then performedin the direction with slower channel change. Using these measurements,one-dimensional interpolation is then performed in the direction withfaster channel change.

FIGS. 5B, 5C and 5D are three other examples of scattered pilot patternswhich can be generated according to this embodiment.

FIG. 6 is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 7A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 7B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 7C is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 7D is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 8 is a diagram of a single antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 9A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 9B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 9C is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 9D is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 10A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 10B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 11A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 11A is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 12A is a diagram of a one antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

FIG. 12B is a diagram of a four antenna irregular diamond latticescattered pilot pattern which can be used in accordance with anembodiment of the present invention.

For the purposes of providing context for embodiments of the inventionfor use in a communication system, FIGS. 13-17 will now be described. Aswill be described in more detail below, the method of the presentinvention can, in one embodiment, be implemented through means of thechannel estimation logic of a conventional OFDM receiver (see channelestimation 96 in FIG. 17).

FIG. 13 shows a base station controller (BSC) 10 which controls wirelesscommunications within multiple cells 12, which cells are served bycorresponding base stations (BS) 14. In general, each base station 14facilitates communications using OFDM with mobile and/or wirelessterminals 16, which are within the cell 12 associated with thecorresponding base station 14. The movement of the mobile terminals 16in relation to the base stations 14 results in significant fluctuationin channel conditions. As illustrated, the base stations 14 and mobileterminals 16 may include multiple antennas to provide spatial diversityfor communications.

A high level overview of the mobile terminals 16 and base stations 14upon which aspects of the present invention may be implemented isprovided prior to delving into the structural and functional details ofthe preferred embodiments. With reference to FIG. 14, a base station 14is illustrated. The base station 14 generally includes a control system20, a baseband processor 22, transmit circuitry 24, receive circuitry26, multiple antennas 28, and a network interface 30. The receivecircuitry 26 receives radio frequency signals bearing information fromone or more remote transmitters provided by mobile terminals 16(illustrated in FIG. 13). A low noise amplifier and a filter (not shown)may cooperate to amplify and remove broadband interference from thesignal for processing. Downconversion and digitization circuitry (notshown) will then downconvert the filtered, received signal to anintermediate or baseband frequency signal, which is then digitized intoone or more digital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). Various modulation andprocessing techniques available to those skilled in the art are used forsignal transmission between the base station and the mobile terminal.

With reference to FIG. 15, a mobile terminal 16 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 16 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals bearing information fromone or more base stations 14. A low noise amplifier and a filter (notshown) may cooperate to amplify and remove broadband interference fromthe signal for processing. Downconversion and digitization circuitry(not shown) will then downconvert the filtered, received signal to anintermediate or baseband frequency signal, which is then digitized intoone or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. The baseband processor 34 is generallyimplemented in one or more digital signal processors (DSPs) andapplication specific integrated circuits (ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are used for signal transmission between themobile terminal and the base station.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation utilizes the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalrecovers the transmitted information. In practice, the IFFT and FFT areprovided by digital signal processing carrying out an Inverse DiscreteFourier Transform (IDFT) and Discrete Fourier Transform (DFT),respectively. Accordingly, the characterizing feature of OFDM modulationis that orthogonal carrier waves are generated for multiple bands withina transmission channel. The modulated signals are digital signals havinga relatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

In operation, OFDM is preferably used for at least down-linktransmission from the base stations 14 to the mobile terminals 16. Eachbase station 14 is equipped with “n” transmit antennas 28, and eachmobile terminal 16 is equipped with “m” receive antennas 40. Notably,the respective antennas can be used for reception and transmission usingappropriate duplexers or switches and are so labeled only for clarity.

With reference to FIG. 16, a logical OFDM transmission architecture willbe described. Initially, the base station controller 10 will send datato be transmitted to various mobile terminals 16 to the base station 14.The base station 14 may use the channel quality indicators (CQIs)associated with the mobile terminals to schedule the data fortransmission as well as select appropriate coding and modulation fortransmitting the scheduled data. The CQIs may be directly from themobile terminals 16 or determined at the base station 14 based oninformation provided by the mobile terminals 16. In either case, the CQIfor each mobile terminal 16 is a function of the degree to which thechannel amplitude (or response) varies across the OFDM frequency band.

Scheduled data 44, which is a stream of bits, is scrambled in a mannerreducing the peak-to-average power ratio associated with the data usingdata scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. Again, thechannel coding for a particular mobile terminal 16 is based on the CQI.In some implementations, the channel encoder logic 50 uses known Turboencoding techniques. The encoded data is then processed by rate matchinglogic 52 to compensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably,Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key(QPSK) modulation is used. The degree of modulation is preferably chosenbased on the CQI for the particular mobile terminal. The symbols may besystematically reordered to further bolster the immunity of thetransmitted signal to periodic data loss caused by frequency selectivefading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. When spatialdiversity is desired, blocks of symbols are then processed by space-timeblock code (STC) encoder logic 60, which modifies the symbols in afashion making the transmitted signals more resistant to interferenceand more readily decoded at a mobile terminal 16. The STC encoder logic60 will process the incoming symbols and provide “n” outputscorresponding to the number of transmit antennas 28 for the base station14. The control system 20 and/or baseband processor 22 as describedabove with respect to FIG. 14 will provide a mapping control signal tocontrol STC encoding. At this point, assume the symbols for the “n”outputs are representative of the data to be transmitted and capable ofbeing recovered by the mobile terminal 16.

For the present example, assume the base station 14 has two antennas 28(n=2) and the STC encoder logic 60 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 60 is sent to a corresponding IFFT processor 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing, alone or in combination with otherprocessing described herein. The IFFT processors 62 will preferablyoperate on the respective symbols to provide an inverse FourierTransform. The output of the IFFT processors 62 provides symbols in thetime domain. The time domain symbols are grouped into frames, which areassociated with a prefix by prefix insertion logic 64. Each of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, pilotsignals known by the intended mobile terminal 16 are scattered among thesub-carriers. The mobile terminal 16, which is discussed in detailbelow, will use the pilot signals for channel estimation.

Reference is now made to FIG. 17 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry72 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time domain to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionlogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency. Examples ofscattering of pilot symbols among available sub-carriers over a giventime and frequency plot in an OFDM environment are found in PCT PatentApplication No. PCT/CA2005/000387 filed Mar. 15, 2005 assigned to thesame assignee of the present application. Continuing with FIG. 17, theprocessing logic compares the received pilot symbols with the pilotsymbols that are expected in certain sub-carriers at certain times todetermine a channel response for the sub-carriers in which pilot symbolswere transmitted. The results are interpolated to estimate a channelresponse for most, if not all, of the remaining sub-carriers for whichpilot symbols were not provided. The actual and interpolated channelresponses are used to estimate an overall channel response, whichincludes the channel responses for most, if not all, of the sub-carriersin the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols.

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

In parallel to recovering the data 116, a CQI, or at least informationsufficient to create a CQI at the base station 14, is determined andtransmitted to the base station 14. As noted above, the CQI may be afunction of the carrier-to-interference ratio (CR), as well as thedegree to which the channel response varies across the varioussub-carriers in the OFDM frequency band. The channel gain for eachsub-carrier in the OFDM frequency band being used to transmitinformation is compared relative to one another to determine the degreeto which the channel gain varies across the OFDM frequency band.Although numerous techniques are available to measure the degree ofvariation, one technique is to calculate the standard deviation of thechannel gain for each sub-carrier throughout the OFDM frequency bandbeing used to transmit data.

FIG. 18 is a block diagram of one embodiment of the present invention.In this embodiment, the present invention is shown being implementedwithin channel estimation logic 96 of FIG. 17 with the conventionalaspects of channel estimation logic 96 being shown in dotted outline forease of reference. Persons skilled in the art will appreciate that thepresent invention could be implemented as a separate logical componentas well.

Shown is time direction channel calculator 127 which performs thecalculation of channel change in the time direction. Frequency directionchannel calculator 129 performs the calculation of channel change in thefrequency direction. As explained above, the preferred calculation isthe inner product of the two pilot assisted channel estimates beingcompared. Though time direction channel calculator 127 is shown as beingillustrated to the right of frequency direction channel calculator 129,this does not mean that the time direction channel calculation isnecessarily to be performed first or that the calculations cannot beperformed simultaneously. Either calculation can be performed first, orboth can be performed simultaneously. Channel direction comparator 131compares the results of the calculations performed by both directionchannel calculator 127 and frequency direction channel calculator 129for the purpose of comparing and ascertaining which channel direction,time or frequency, changes slower. Channel direction selector 133selects which of the two directions changes slower. Block 135 isutilized to interpolate, first in the direction of slower change, andthen in the direction of faster change, in accordance with conventionalmeans.

In operation, time direction channel calculator 127 receives two pilotassisted channel estimates and performs the calculation of channelchange in the time direction. Frequency direction channel calculator 129performs the calculation of channel change in the frequency directionthough these two calculations can be performed in different order orsimultaneously. Channel direction comparator 131 compares the results ofthe calculations performed by both direction channel calculator 127 andfrequency direction channel calculator 129 and compares which channeldirection, time or frequency, changes slower. Channel direction selector133 selects the direction of slower change and interpolation is thenperformed by block 135 in that direction first, and then in thedirection of faster change in accordance with conventional means.

FIGS. 13 to 18 each provide a specific example of a communication systemor elements of a communication system that could be used to implementembodiments of the invention. It is to be understood that embodiments ofthe invention can be implemented with communications systems havingarchitectures that are different than the specific example, but thatoperate in a manner consistent with the implementation of theembodiments as described herein.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

1. A method comprising: receiving channel estimates for four pilotsymbols in a scattered pilot pattern in time-frequency; calculating thechannel response for the pilot symbols in both a first direction and asecond direction; determining whether the channel changes more slowly inone direction than the other; interpolating in the direction of slowerchannel change.
 2. The method of claim 1 further comprisinginterpolating in the direction of faster channel change.
 3. The methodof claim 2 wherein the step of interpolating in the direction of fasterchannel change is performed using the result from the step ofinterpolating in the direction of slower channel change.
 4. The methodof claim 1 wherein the channel changes are calculated by performing aninner products operation.
 5. The method of claim 1 wherein the firstdirection is a time direction and the second direction is a frequencydirection.
 6. The method of claim 1 wherein the first direction is afrequency direction and the second direction is a time direction.
 7. Themethod of claim 1 wherein the scattered pilot pattern is a regulardiamond lattice.
 8. The method of claim 1 wherein the scattered pilotpattern is an irregular diamond lattice.
 9. The method of claim 1wherein the scattered pilot pattern is kite shaped.
 10. An OFDM receivercomprising: one or more receive antennas; the OFDM transmitter beingadapted to receive channel estimates for four pilot symbols in ascattered pilot pattern in time-frequency, calculate channel changes forthe pilot symbols in a first direction and a second direction, andinterpolate in the direction of slower channel change.
 11. The OFDMreceiver of claim 10 wherein said receiver is further adapted tointerpolate in the direction of faster channel change.
 12. The OFDMreceiver of claim 11 wherein interpolating in the direction of fasterchannel change is performed using the result from interpolating in thedirection of slower channel change.
 13. The OFDM receiver of claim 10wherein the channel changes are calculated by performing an innerproducts operation.
 14. The OFDM receiver of claim 10 wherein the firstdirection is a time direction and the second direction is a frequencydirection.
 15. The OFDM receiver of claim 10 wherein the first directionis a frequency direction and the second direction is a time direction.16. The OFDM receiver of claim 10 wherein the scattered pilot pattern isa regular diamond lattice.
 17. The OFDM receiver claim 10 wherein thescattered pilot pattern is an irregular diamond lattice.
 18. The OFDMreceiver claim 10 wherein the scattered pilot pattern is kite shaped.19. A method of interpolation using a set of four pilot symbols in ascattered pilot pattern in time-frequency wherein the set of four pilotsymbols comprise first and second pilot symbols on a common sub-carrierfrequency, spaced in time, and third and fourth pilot symbolstransmitted on different sub-carriers on a common OFDM symbol period,the method comprising: determining a first channel change between thefirst and second pilot symbols; determining a second channel changebetween the third and fourth pilot symbols; determining which of thefirst and second channel change is slower; if the first channel changeis slower, interpolating using the first and second pilot symbols togenerate a channel estimate for the common sub-carrier frequency at thecommon OFDM symbol period, and then using the channel estimate insubsequent interpolations to determine other channel estimates; and ifthe second channel change is slower, interpolating using the third andfourth pilot symbols to generate a channel estimate for the commonsub-carrier frequency at the common OFDM symbol period, and then usingthe channel estimate in subsequent interpolations to determine otherchannel estimates.
 20. A method of inserting pilot symbols into OFDMsub-frames for transmission by a plurality of transmitting antenna, theOFDM sub-frames having a time domain and a frequency domain, each OFDMsub-frame comprising a plurality of OFDM symbols, the method comprising:for each sub-frame, defining a set of at least two OFDM symbols none ofwhich are consecutive that are to contain pilot symbols; at eachantenna, inserting pilot symbols in each of the set of at least two OFDMsymbols in a scattered pattern that does not interfere with thescattered pattern inserted by any other antenna.
 21. The method of claim20 wherein the scattered patterns of the antennas are identical butstaggered in the frequency domain.
 22. The method of claim 20 whereinthe scattered patterns are regular diamond lattices.
 23. The method ofclaim 20 wherein the scattered patterns are irregular diamond lattices.24. The method of claim 20 wherein the scattered patterns are kiteshaped.
 25. The method of claim 20 wherein the number of OFDM symbolsper OFDM sub-frame is
 7. 26. The method of claim 20 wherein each OFDMsub-frame comprises 8 sub-carriers.
 27. The method of claim 20 whereineach OFDM sub-frame comprises 10 sub-carriers.
 28. The method of claim20 wherein each OFDM sub-frame comprises 12 sub-carriers.
 29. The methodof claim 20 wherein the pilot symbols are inserted in each of the set ofat least two OFDM symbols in pairs.
 30. The method of claim 20 whereinthe pilot symbols are inserted in each of the set of at least two OFDMsymbols in quartets.
 31. The method of claim 20 wherein the number ofOFDM symbols per OFDM sub-frame is odd.
 32. The method of claim 20wherein the scattered patterns are cyclically offset, both in a timedirection and in a frequency direction.
 33. The method of claim 20wherein each sub-frame comprises first, second, third, fourth, fifth,sixth and seventh OFDM symbols in sequence, and wherein said set of atleast two OFDM symbols comprises the first and fourth OFDM symbols.